A grooved pulley or sheave incased in a frame or shell which is provided with a hook, eye, or strap, by which it may be attached to an object. It is used to change the direction of motion, as in raising a heavy object that can not be conveniently reached, and also, when two or more such sheaves are compounded, to change the rate of motion, or to exert increased force; -- used especially in the rigging of ships, and in tackles.